The invention relates to a method of positioning a welding torch with a welding wire at the center of a desired path of a welding seam, and a method of detecting a short circuit between the welding wire and a workpiece on completion of a welding process, as well as a welding device for a robot application.
A method is known from U.S. Pat. No. 4,249,062 A, in which a switching element of a control system activates a sub-routine to operate a changeover from one energy source or welding current source to another energy source or voltage source. The positioning process to determine the seam centre is then operated by means of this energy source. On completion of the sub-routine, the switching element is switched back so that a welding process can now be run with the welding current source. The disadvantage of this system is that an additional current source and a switching element must be provided for determining the coordinates of the seam and a changeover has to be operated between the two energy sources depending on the process being run.
A method and a device for positioning a welding torch at the centre of a seam is known from patent specification DE 35 45 505 A1, in which the welding torch is positioned on a predetermined or programmed seam centre by a robot arm, which is followed by an alignment process. The robot arm displaces the welding torch in one direction until a short circuit occurs between the welding torch, in particular a gas nozzle, and a seam edge of the workpiece and the robot arm then moves the welding torch in the opposite direction until another short circuit occurs between the welding torch, in particular the gas nozzle, and the workpiece. A control system then evaluates the displacement path to work out a seam mid-point on which the welding torch will be positioned by the robot. The contour of the workpiece is mechanically scanned by sensing the contact between the adjacent seam edges of the seam reference point of a workpiece and the coordinates of the seam root point are computed from this data and from the displacement path. To this end, the welding torch is pivotably mounted on the robot arm and has a switch plate which lifts on contact with the seam edge, pivoting the welding torch pipe out from the welding torch, and a switching signal is transmitted to the control system.
The disadvantage of this system is that the welding torch has to be specially designed to be able to determine the coordinates mechanically, which means that not every welding torch can be used for this purpose.
Another method and a device for mechanically determining the coordinates of a seam centre is known from patent specification DE 298 16 729 U1, in which an auxiliary current source is activated by a control system. The output of the auxiliary current source is defined so that, when a short circuit occurs between the welding wire and the workpiece or welding seam edge, an arc is not struck when the alignment process is taking place. The coordinates are determined, also by displacing the welding torch and the welding wire to the left and right accordingly, so that when a short circuit occurs between the welding wire and the workpiece, a switching device, in particular a relay, is activated and an indication that a short circuit has occurred is transmitted to the control system via the switching element coupled with the switching system. The welding torch and the welding wire are then displaced in the opposite direction. The coordinates of the seam centre can then be calculated.
The disadvantage of this system is that an additional auxiliary current source is needed in order to determine the coordinates of the seam and is provided in the form of an add-on component to the welding device, which means that an extra switching device is needed in order to provide a connection to the welding device.
The underlying objective of the invention is to propose a method for positioning a welding torch and a welding wire in the middle of a desired run of welding seam and a method for detecting a short circuit between workpiece and welding wire on completion of a welding process, as well as a welding device or a welding system for a robot application, in which the alignment can be found without the need for additional devices or components.
In a method of positioning a welding torch and a welding wire of a welding device at a center of a desired path of a welding seam, in which the welding torch is positioned by means of a robot arm on a predetermined or programmed position between two seam edges of a workpiece and an alignment process is run whereby the robot arm moves the welding torch in one direction until a short circuit occurs between the welding torch or the welding wire and one of the seam edges, and the welding torch is then displaced by the robot arm in the opposite direction until another short circuit occurs between the welding torch or the welding wire and the other one of the seam edges but no arc is ignited between the welding torch or welding wire and the workpiece when a short circuit occurs, after which a control system evaluates the displacement path in order to calculate the seam center on which the welding torch or welding wire is positioned by the robot arm, the present invention uses a single current source having a power component supplying cower for the welding process and the alignment process, the control system reduces the power output of the power component by selecting a xe2x80x9cFind positionxe2x80x9d function to the degree that a chance of resistance is detected on the basis of the power applied to output terminals of the power component when a short circuit occurs and, to detect the short circuit, discharge capacitors between the output terminals and of the power component are loaded by an output power of the power component with a low capacity so that ignition of an arc is prevented when a short circuit occurs.
The advantage of this system method is that by controlling the power component of the welding device accordingly, the output power can be reduced so that no arc is struck when the welding wire comes into contact with the workpiece, obviating the need for additional devices or components, such as an auxiliary current source. Another advantage resides in the fact that the user or welder can activate the power component automatically, simply by activating a function, and welding parameters already set up for a welding process will not be lost and the welding process can therefore be initiated after the alignment process. Another advantage is that by reducing the output power of the power component, in particular a bridge circuit, the pulse width is already so small that it prevents an arc being inadvertently ignited when a short circuit occurs. The power supply of a secondary circuit is set up in such a way that the power lost by the components is compensated and only a low charge is applied to the suppression and/or discharge capacitors disposed on the output terminals.
This ensures that only a small quantity of energy is provided for the purpose of detecting a change in resistance or a short circuit. Another significant advantage is the fact that this method of activating the power component can be operated by appropriate software programmes and the system can therefore be operated with older welding devices, simply by updating the software.
According to another aspect, this invention provides a method of detecting a short circuit between a welding wire and a workpiece on completion of a welding process, a no-load voltage being applied to the welding wire, which breaks down in case of a short current, and the break down being detected by a control system, wherein the welding device uses a single current source having a power component supplying power for the welding process and for detecting a short circuit on completion of the welding process, the power component is activated by the control system on completion of the welding process and after a predeterminable time has elapsed so that, when the short circuit is detected between the welding wire and the workpiece, a minimal output power is supplied or the power component is switched off, thereby preventing ignition of an arc and, to detect the short circuit, discharge capacitors between the output terminals are loaded by an output power of the power component with a low capacity so that ignition of an arc is prevented when a short circuit occurs.
The advantage of this approach is that by minimizing the output power when checking for a change in resistance or a short circuit at the end of the welding process, no damage can be caused to the workpiece and the welding wire, because ignition of an arc is prevented or is not possible. Another advantage resides in the fact that the minimal output power applied during a checking process means that persons working with the welding torch or workpiece are not at risk.
The invention comprises a welding torch and a welding wire, a robot arm carrying the welding torch for positioning the welding torch and welding wire on a predetermined or programmed position between two seam edges of a workpiece in an alignment process in which the robot arm moves the welding torch in one direction until a short circuit occurs between the welding torch or the welding wire and one of the seam edges, and the welding torch is then displaced by the robot arm in the opposite direction until another short circuit occurs between the welding torch or the welding wire and the other one of the seam edges but no arc is ignited between the welding torch or welding wire and the workpiece when a short circuit occurs; a single current source having a power component supplying power for the welding process and the alignment process; a controllable xe2x80x9cFind positionxe2x80x9dfunction controlling the power component with a minimal output power when a short circuit occurs and ignition of an arc is prevented; and, to detect the short circuit, discharge capacitors connected to output terminals of the power component at a secondary side of the current source, the discharge capacitors being loadable by an output power of the power component with a low capacity whereby ignition of an arc is prevented when a short circuit occurs.
The advantage here is that, simply by setting and activating a function and a welding parameter, the power component can be controlled separately by the control system. A function of this type can easily be set up to run with existing welding devices because it can be achieved simply by a software update, requiring no modifications of the hardware.